1. Field of the Invention
The present invention relates to force sensing resistors.
2. Background Art
A pressure transducer may generate a signal indicative of the amount and location of pressure applied on the transducer. A pressure transducer enables a user to provide input for portable electronic devices like personal digital assistants, telephones, remote controls, etc. Such portable devices are trending toward increasing functionality without increasing physical size. The increased functionality is accompanied by a need for more user input and interaction. This leads to demand for intuitive input devices such as touch surfaces useable in the portable devices.
A construction for a pressure transducer includes one or more force sensing resistors (FSRs). Various FSRs have been described such as in U.S. Pat. Nos. 4,314,227; 4,314,228; and 4,489,302. An FSR is useful for adding to the utility of touch interfaces. By sensing force, an FSR could be used, for example, to determine when the front glass surface of a phone is touched and then subsequently released.
A basic FSR includes two substrates (i.e., membranes) separated by a thin air gap. A spacer around the edges of the substrates and the rigidity of the substrates maintain the air gap. One substrate includes electrically conductive traces arranged into two sets of interdigitated fingers. The traces of one finger set are electrically distinct from the traces of the other finger set. The other substrate includes FSR ink. When a force is applied to the FSR such that the substrates are pressed together, the FSR ink electrically connects traces of the finger sets together with a resistance dependent on the applied force.
When no force is applied to the FSR, the air gap remains open and the FSR is in an open circuit. Only with the application of a small force does the air gap close and, consequently, the FSR has a finite resistance. This has been viewed as advantageous because the open circuit state is a distinct and unambiguous “not touched” signal. This means that the FSR does not require continuous measurement in order to detect touch. As such, a measurement processor can remain in a power saving sleep mode whereby an initial force takes the FSR from the open circuit state to some finite resistance which can be used to provide a wake-up signal to the measurement processor.
However, in portable devices such as cell phones, mp3 players, etc., where thinness is critical, it can be difficult to design mechanics that successfully maintain the air gap during a non-touched state. An FSR whose substrates touch even in the absence of an externally applied force is a “pre-loaded” FSR.